1. Field of the Invention
The present invention relates to a headlight device for a motorcycle, which is designed to permit a motorcycle rider to gain a relatively large illuminated field of view forwardly of the motorcycle while the latter makes the curve during a night drive.
2. Description of the Prior Art
As is well known to those skilled in the art, the motorcycle, when ready to change the direction of travel either leftward or rightward with respect to the direction of straightforward run thereof while traveling at a certain speed, is tilted or banked either leftward or rightward at a bank angle. However, considering that the headlight used in most motorcycles now in use is fixed in position to the motorcycle frame structure and, therefore, the region of illumination cast by the motorcycle headlight is correspondingly tilted as the motorcycle is banked, the amount of light distributed by the motorcycle headlight in an area inwardly of the direction of movement of the motorcycle, in which the motorcycle rider's eyes are focused, particularly during the cornering at night decreases, with the rider's field of view forwardly of the direction of travel narrowed consequently.
More specifically, referring to FIG. 18 showing the forward field of view available to the motorcycle rider during the straight forward drive at night, the region of illumination A cast by the motorcycle headlight upon an area of the road surface forwardly of the motorcycle spreads in leftward and rightward directions of the motorcycle, which are parallel to the horizontal datum line HL. However, as best shown in FIG. 19, when the motorcycle makes the curve in a leftward direction along a curved lane 50 as shown by the arrow-headed line P therein, as the frame structure is tilted in the corner, the illumination region A cast similarly by the motorcycle headlight on an area of the road surface tilts downwardly leftward as compared with the illumination region A exhibited during the straight forward run of the motorcycle. Therefore, a forward portion of the road surface inwardly of the turning direction of the motorcycle as indicated by the dotted circle B in FIG. 19, where the motorcycle rider's eyes are generally focused during the cornering encompasses less portion of the illumination region A than that during the straight forward run, with the consequence that the rider's field of view forwardly of the direction of cornering is virtually narrowed.
In view of the foregoing, the assignee of the present invention has suggested in, for example, the Japanese Laid-open Patent Publication No. 2001-347977, published Dec. 18, 2001 by the same assignee, the headlight device for a motorcycle, which is effective to expand the rider's field of view during the cornering at night. According to this published patent document, the motorcycle headlight device is so designed and so configured that based on the bank angle of the motorcycle detected by a bank angle detecting unit, a rotation control unit may control a drive unit to cause the latter to pivot a combination of a lens and a light emitting element, forming respective parts of a headlamp module, about the optical axis of the headlamp module in a direction counter to the direction of tilting of the motorcycle over an angle corresponding to the detected bank angle of the motorcycle. With this known headlight device, a forward portion of the road surface inwardly of the turning direction of the motorcycle, where the motorcycle rider's eyes are generally focused during the cornering, can encompass more light distribution illuminated by the headlamp module, with the rider's field of view forwardly of the direction of cornering increased consequently.
However, the motorcycle headlight device such as discussed above has been found having a problem. Specifically, the bank angle detecting unit calculates the bank angle of the motorcycle on the basis of the traveling velocity and the angular velocity of turn of the motorcycle about a vertical axis of the motorcycle. The angular velocity of turn of the motorcycle about the vertical axis corresponds to an angular velocity of turning in a horizontal plane, defined by the angle of steering of the motorcycle handlebar. Accordingly, the bank angle detected, i.e., calculated by the bank angle detecting unit corresponds to the bank angle exhibited during the steady turning at a predetermined bank angle.
However, considering that during the cornering it is usual for the motorcycle rider to tilt the motorcycle, followed by turning of the motorcycle handlebar in a direction in which he or she desire to drive, the bank angle of the motorcycle based on the angular velocity about the vertical axis of the motorcycle and the traveling velocity tends to be calculated with a certain length of time delayed from the moment, at which the actual banking of the motorcycle starts. For this reason, the headlamp module is rotated or pivoted with a certain length of time delayed relative to the banking of the motorcycle and, therefore, at an initial stage, in which the motorcycle starts banking, distribution of light cast by the headlamp module would not result in a proper illumination sufficient to allow the motorcycle drive to enjoy a relatively large field of view forwardly of the motorcycle. This delay in operation of the headlamp module is indeed prominent particularly where the motorcycle is maneuvered with legerity such as occurring during traveling in, for example, a zigzag course.
It is also well known that when the motorcycle is banked during the cornering, the height of the headlamp module above the road surface lowers as compared with that during the straight forward run with the motorcycle held in an erected position. Once the height of the headlamp module is so lowered, the distance of reach of the light cast by the headlamp module in a direction forwardly of the motorcycle is reduced a quantity corresponding to the height so lowered and, accordingly, during the cornering at night, the headlamp module is unable to illuminate the road surface a distance further away from the motorcycle as is the case with the straight forward travel.